This is for using widgets that don't provide a QtDesigner plugin in projects that use .ui files (QtDesigner files). So you add the base class (e.g. QWidget) in QtDesigner and promote it to the subclass. All this changes is what class name is used in the generated code file and thus what class gets instantiated during runtime. Nothing changes in QtDesigner, though.

[quote author="Flurite" date="1346021970"]So I promoted a widget to a class called mypanelopengl.h, but I didn't see a new function in the class. Does it only happen when I build the project, or am I doing something wrong?[/quote]

Does the header file mypanelopengl.h exist and contain a class with new functions?

[quote author="Flurite" date="1346021970"]So I promoted a widget to a class called mypanelopengl.h, but I didn't see a new function in the class. Does it only happen when I build the project, or am I doing something wrong?[/quote]

I think you're talking about auto-complete for the funtion inside that header?